Jeff Bezos is notoriously private, rarely agreeing to interviews and never talking politics. His only widely-known interests besides leading online retail giant Amazon have been space exploration and science.

All of this makes Friday's announcement all the more unexpected: Bezos and his wife MacKenzie have cut a check for $2.5 million towards equal marriage rights.

As the Times reported, it was an email from a former Amazon employee from the company's early days that prompted Bezos and his wife to publicly back equal rights. Jennifer Cast, a lesbian mother of four, sent Bezos a note last Sunday asking for $100,000 towards Referendum 74. She received a reply two days later confirming his support and $2.5 million donation.

Bezos joins a growing list of Forbes 400 rich list members backing the legalization of same-sex marriage across the country. As my colleague Ryan Mac has reported, Silicon Valley's tech billionaires have been vocal equal rights supporters as far back as 2008 and California's Proposition 8.

Billionaire media mogul David Geffen, who is openly gay, also backed the movement against Proposition 8 in 2008, giving $100,000 to the No On 8 campaign.

Other openly gay billionaires who've long backed the legalization of same-sex marriage include Jon Stryker, heir to the Stryker Corp. medical supply fortune. Stryker gave $602,000 in 2010 to the Gay & Lesbian Victory Fund.

Peter Thiel, the PayPal and Facebook billionaire, is also gay and has helped fund right-leaning LGBT group GOProud. In 2010, he hosted a GOProud event at his New York home that caused some consternation on the gay blogosphere for its host, Ann Coulter, who isn't exactly known for her liberal views on marriage rights.

Some Forbes 400 rich list members have become backers of equal rights as a show of solidarity to their gay sons and daughters. Perhaps the most prolific donor to the cause is not particularly well-known outside New York finance circles: hedge fund billionaire Paul Singer, who has given more than $10 million and counting.

Singer avoids the limelight more so than Bezos, but agreed to talk to the NYT's Frank Bruni in June when he launched a pro-gay rights Super PAC. The Republican billionaire pledged $1 million to start the group, and told Bruni his commitment stems from a cause close to home. He has a gay son, who married his partner in Massachusetts in 2o04.

Another billionaire backer of equal rights: Peter Lewis of insurance giant Progressive. Lewis himself is progressive in nature as well as name as the country's highest-profile billionaire supporter of marijuana legalization. In an interview last year, he told Forbes his focus nowadays is drug reform, but the lifelong Democrat has donated to LGBT groups in the past. His son Jonathan is gay and a major Democratic fundraiser who gave $250,000 to the Gay & Lesbian Victory Fund in 2008.

Real estate developer and conservative political donor Steven Roth is another billionaire with a personal stake in the issue: like Lewis and Singer, he has a gay son. He and wife Daryl both gave $16,800 to support the campaigns of Republican New York state senators who backed same-sex marriage.

Fellow Republican donor and billionaire Steven Cohen also backed New York’s gay marriage initiative, as did Singer. Another notable super-rich supporter of the successful New York equal rights bid: the city's billionaire Mayor Mike Bloomberg, a vocal LGBT advocate for years.

The next generation of Forbes 400 members is getting in on the act too. Laura Ricketts, daughter of conservative Chicago Cubs billionaire Joe, is one of President Obama’s bundlers, or super-fundraisers, tapping her network of wealthy, influential friends, colleagues and peers to raise an unlimited amount for his 2012 campaign.

Ricketts took her equal rights advocacy a step further this month as one of the founding members of LPAC, a super PAC focused on lesbian rights. On the day of LPAC's launch, July 11, Ricketts and her team already had $200,000 in commitments, and they're fundraising in earnest, with big-name lesbians including Glee's Jane Lynch and tennis legend Billie Jean King on board.

This week, LPAC chair Sarah Schmidt spoke of the group's success since it launched, and the importance of straight allies -- like Jeff and MacKenzie Bezos. "I hoped that we would be able to connect with thousands of women and men, gay and straight, across the country," she said. "And we did!"